Fix a bug introduced by e9dbfae5, which prevents event_subsystem fromever being released.

Ref_count was added to keep track of subsystem users, not for countingevents. Subsystem is created with ref_count = 1, so there is no need toincrement it for every event, we have nr_events for that. Fix this bytouching ref_count only when we actually have a new user -subsystem_open().